The ability of a geological formation to allow the passage of fluids is dependent upon the size of the pores, their connectivity (permeability) and the properties of the fluid. The effective permeability also depends upon the relative saturations of the various fluids within the pores. Within hydrocarbon reservoirs, the permeability affects the flow path of both formation fluids and injected fluids within the reservoir. It is beneficial to know the reservoir permeabilities to optimize production strategies.
Various attempts have been made to trace fluid flow within the reservoir using tracers placed into injection wells and detected during production. U.S. Pat. No. 6,645,769 describes such a technology. The use of these methods is limited by the fact that tracers can only be detected in the production well and at least two wells must be drilled.
Other methods propose the use of acoustic properties of injection fluids in order to trace their spatial distribution through time (U.S. Pat. Nos. 4,479,204; 4,969,130; 5,586,082; 6,438,069). Such methods are limited by the fact that the acoustic properties are not always a reliable measure of the fluid composition.
The object of the present invention is to overcome the limitations of the above mentioned methods by injecting a fluid that can be sensed by resistivity mapping techniques. The electrical resistivity is a parameter which is highly dependent on the fluid type. Resistivity mapping has been used for hydrocarbon prospecting as described in U.S. Pat. Nos. 4,617,518; 4,633,182; 6,603,313; 5,770,945. Its use for reservoir monitoring purposes is described in U.S. Pat. No. 6,739,165.